Field of the Invention
The present invention concerns a device for generating a frequency-modulated signal.
Certain systems such as altimeters or pulse compression radars emit a frequency-modulated signal according to a predetermined modulation law. The control oscillators that equip them must follow this law with the greatest possible precision and stability.
Several processes for obtaining a frequency-modulated signal of a limited duration are known; they operate as follows:
by modulating directly a frequency oscillator controlled by a voltage, called voltage control oscillator (VCO), and by possibly completing by a feed-back loop; PA1 by mechanically controlling an oscillator element, in general a capacitor armature; PA1 by using a dispersive line of predetermined characteristics, fed by a brief pulse, which allows the obtaining of a signal of a limited duration; PA1 in the case of a linear modulation, by combining a phase loop and a delay line of which the group delay is known, such a process being described in the article of W. J. CAPUTI, "Stabilized linear FM generator" which appeared in IEEE trans. on A.E.S., September 1973. PA1 a digital generator that, from a clock signal of period 1/f.sub.H and an external control signal, delivers on the one hand an output signal s(t) of which each alternance presents a variable duration (N.sub.o T, N.sub.1 T, . . . , N.sub.k T, N.sub.n T) that is an integer multiple of the clock period, and, on the other hand, an error signal (S.sub.e) representing the errors of the said output signal s(t) with respect to the theoretical signal (S.sub.t) modulated according to the predetermined law; and, PA1 error correction means supplied with the said output signal and controlled by the error signal.
The carrying out of the processes described herein-above requires the use of high-quality analog components and delicate operation if a good stability with temperature and ageing is desired.